1. Field of the Invention
The present invention relates generally to a method and an apparatus for estimating a channel in a wireless communication system, and in particular, to a method and an apparatus for estimating a channel using phase compensation in a wireless communication system.
2. Description of the Related Art
Recently, various studies and research have been actively performed for the Orthogonal Frequency-Division Multiplexing (OFDM) scheme which is a used in a mobile communication system to transmit data at high speed through a wire/wireless channel. The OFDM scheme is a scheme in which data is transmitted using a multi-carrier, and a type of Multi Carrier Modulation (MCM) scheme in which a series of input symbols is converted into parallel data and each piece of this data is transmitted through a number of subcarriers which are kept in an orthogonal relationship to each other.
The OFDM scheme can be widely used in digital transmission technologies such as Digital Audio Broadcasting (DAB), digital television, Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), and high-speed packet data system. The OFDM scheme had not been widely used in the past due to the complexity of its hardware, but has been recently realized as digital signal processing technologies were developed, such as Fast Fourier Transform (FFT) technology, Inverse Fast Fourier Transform (IFFT) technology, and the like.
In the OFDM system, a modulation process and a demodulation process in a transmitter and a receiver correspond to an Inverse Discrete Fourier Transform (IDFT) process and a Discrete Fourier Transform (DFT) process, respectively. These processes can be efficiently realized using the IFFT and the FFT whereby the transmitter of the OFDM system carries a sending signal on plural subcarriers through the IFFT, and the receiver receives the OFDM signal through a wireless channel and divides the received OFDM signal through the FFT.
Similar to other wireless communication systems using wireless channels, the receiver of the OFDM system estimates the impulse response of the wireless channel which represents the feature of the wireless channel from the transmitter to the receiver, and performs channel compensation on the received signal through the estimated impulse response. In general, the receiver of the OFDM system uses an equalizer such as the Zero-Forcing (ZF) scheme and the Minimum Mean Square Error (MMSE) scheme in order to compensate the channel.
The MMSE scheme is representative among the conventional channel estimation scheme described above, as is Wiener Filtering. The MMSE scheme estimates the channel using a Least Square (LS) technique, obtains a weight based on the criterion that minimizes the channel estimation error, and multiplies the weight by the LS estimation value. Therefore, it is well known as an ideal channel estimation scheme.
In the MMSE scheme, the weight is obtained as follows. First, a correlation matrix is calculated between pilot signals different from each other in the time and the frequency, adds a reciprocal number of a Signal-to-Interference and Noise Ratio (SINR) and the diagonal components in the correlation matrix, and calculates an inverse matrix of the result. Then, a correlation matrix is calculated between data to be estimated and the pilot signals, and multiplies the correlation matrix by the obtained inverse matrix in order, thereby being able to obtaining the weight.
Channel estimation using the Wiener Filtering is very effective. However, since it should continuously calculate the correlation matrix for the changing channel in order to obtain the weight as described above and calculate the inverse matrix every time, the complexity is very high. Therefore, it is difficult to use this scheme for a system in which the channel should be estimated in real time. In a scheme proposed to reduce complexity, the complexity of channel estimation using the Wiener Filtering is taken into consideration, so that a correlation matrix is obtained for a representative channel in a case of the channel estimation scheme of a base station in a system such as the Mobile WiMax. Further, the channel estimation is performed by performing an interpolation or an average in the frequency axis and ID N-tap sliding Wiener filtering in the time axis.
However, in the channel estimation scheme in the Mobile WiMax described above, the phases are shifted for each frequency when there are plural timing offsets. Therefore, the performance of channel estimation may be degraded when interpolation or averaging is performed. Further, even the Wiener Filtering in the time axis is degraded due to the complexity in calculation of the inverse matrix and due to discrepancies between the correlation matrixes of the actual and representative channels. Therefore, there is required a channel estimation scheme which is able to prevent the degradation in performance due to the interpolation and the average in the channel estimation and to solve the complexity of the Wiener filter.